This invention relates to thermally stable zeolites particularly zeolites of the faujasite type which have superior thermal and hydrothermal stability. Faujasite type zeolites have been widely employed in catalytic processes such as processes for the conversion of hydrocarbons and are generally well known. The patent and journal literature is extensive.
In all of these catalyst systems the faujasite type zeolites are produced in the sodium form, that is, the various exchange sites are satisfied by sodium. Such zeolites are described in U.S. Pat. Nos. 2,882,244, usually referred to as X zeolite, and 3,216,789 referred to as a Y zeolite, and also in 3,446,727.
To produce useful catalysts the sodium content must be reduced and this is accomplished by exchange with cations such as ammonium, or hydrogen or polyvalent cations such as alkaline earth and rare earth cations. Commonly, the sodium content of the zeolites employed in catalysts are on the order of about three to about five percent expressed as Na.sub.2 O. The difficulty with such catalysts are that they have inferior hydrothermal stability. When such zeolites are incorporated in a matrix, and are employed in catalytic processes, they are subjected in a cycle of hydrocarbon conversion and regeneration at high temperatures in the presence of steam and they become severely deactivated. Such catalysts have inadequate thermal and hydrothermal stability.
It has been found in the prior art that sodium zeolites of the faujasite type must be exchanged to produce a product with sodium levels substantially lower than about 1% by weight expressed as Na.sub.2 O on a volatile free basis in order that they have adequate hydrothermal stability. This has been accomplished by exchanging the sodium with a cation such as ammonium ion employing a hot solution of an ammonium salt.
The prior art relating to the production of such low sodium zeolites of the faujasite type, referred to in the prior art as ultra-stable zeolites, is extensive and the following U.S. Pat. Nos. are illustrative: Maher et al. 3,293,192 and 3,402,996; Hansford 3,354,077; Sherry 3,677,698.
Maher et al. supra exposes the zeolite to dry calcination temperatures of 1000.degree. F and Ward, 3,781,199 and 3,867,277 carries out the calcination in the presence of steam.
The reduction of the sodium level to below 1% Na.sub.2 O in the above prior art, requires the partial exchange of the sodium at substantially lower temperatures of about 100.degree. C before exposing the zeolite to the higher temperature conditions. The resultant partially exchanged faujasite must then be further exchanged to reduce the sodium levels to less than 1% expressed as Na.sub.2 O.
The resultant zeolite is profoundly altered in crystal structure as is evidenced by substantial change of the a.sub.o lattice constant, see Maher et al supra.
Sherry et al. U.S. Pat. No. 3,677,698 teaches that to reduce the original sodium content of Y zeolite by up to 90% without impairment of crystallinity, the Y zeolite should first be exchanged at a relatively low temperature of 215.degree. F or less to remove 10 to 75% of the original sodium content of the Y zeolite before exposing the partially exchanged zeolite to a further exchange at a higher temperature. Sherry, employing rare earth cations in the exchange process carry out the subsequent exchange at a temperature of 500.degree. F to reduce the Na content to less than 1%.